Method and control system for applying solder flux to a printed circuit

ABSTRACT

A first embodiment includes a control system which operates a spray gun to apply an even coating of flux to a circuit board or a plurality of different length circuit boards irrespective of the speed that the circuit board(s) move through a coating chamber. In a second embodiment, the control system pulses the spray gun on and off with a pulsing pattern that coats a plurality of zones of a circuit board with spray patterns of different length and thickness.

FIELD OF THE INVENTION

This invention relates to the field of fabricating printed circuitboards and more particularly to a method and control system for applyingsolder flux to the circuit boards with a spray gun prior to solderingleads of circuit components to the board.

BACKGROUND OF THE INVENTION

The production of circuit boards typically includes loading the circuitboards onto a conveyor line and transferring them down the conveyor lineto a station where the leads of electrical components are inserted intothe holes through the circuit board by a machine and/or manually. Theboards are then transferred to an edge handling conveyor which transfersthe boards through a flux application station where solder flux isapplied to the circuit boards so that the leads of the electricalcomponents can later be soldered to the metallized areas of the boardwith a high quality solder bond. After moving through the fluxapplication station, the boards are transferred through a preheat zoneto flash off the solvents from the flux and to preheat the board tominimize thermal shock from contact with solder wave. Continuing theprocess, the board is transported through a wave solder machine wherethe board moves over a wave of solder. The solder is drawn or forced upinto through-holes containing the leads of the electrical components andsolder bonds between the leads and the metallized sections of the boardare formed. After leaving the wave solder machine, the board is sentthrough a cleaning machine to remove the residue left from the flux, ifnecessary. The effectiveness of the flux application, the type of fluxbeing applied, the need to clean the board subsequent to soldering, andthe need to clean the coating chamber in which the solder flux isapplied, each present problems which are addressed by the presentinvention.

As described in U.S. Pat. No. 5,415,337, assigned to Nordson Corporationof Westlake, Ohio, the assignee of the present invention, which isincorporated in its entirety herein, low-solid fluxes, or "no-cleanfluxes", which contain small amounts, e.g., about 1% to 5% by weight ofsolids (activator and vehicle) and the remainder liquid solvent, such asisopropyl alcohol, are being increasingly used by circuit boardmanufacturers in an effort to eliminate the need to clean the circuitboards after soldering. Because of the small amount of solids withinno-clean fluxes, the amount of residue left on the board issignificantly reduced, as compared to the amount of residue remainingafter the use of conventional rosin-based fluxes. These low-solid, noclean fluxes are particularly attractive because, as their name implies,and flux cleaning of the circuit boards after soldering is not required,which results in a significant cost savings.

The application of the no-clean fluxes using the control systemdescribed in U.S. Pat. No. 5,415,337 has sometimes lead to problemsbecause the controller is difficult to program for boards of varyinglength, for boards requiring more than one spray profile, for varyingspeeds of the conveyor, for multiple boards carried on pallets, and forspraying selected sections of the boards.

The problem with the control system described in U.S. Pat. No. 5,415,337is that the control system does not always initiate the spray cycle forboards or sometimes initiates the spray cycle when a board is not inplace. The latter problem generates overspray within the coatingchamber. This overspray wastes flux coating material and results in thechamber and the conveyor becoming covered with the flux coating materialand being difficult and time consuming to clean. Also, the overspraytends to clog up the control sensor which signals the location of thecircuit board to the control system. This is detrimental because thespray is then turned on when the board is not in the proper location.This exacerbates the problem by generating even more overspray andcreating an even greater mess which requires more frequent cleaning.Whenever the system is cleaned, the entire line must be shutdown,further increasing the overall manufacturing costs of the circuitboards.

Therefore, there is a need for an easily programmed controller able tocontrol a spray gun for applying solder fluxes, and particularlylow-solid fluxes, to circuit boards so that an even coating of thesolder flux is applied to the entire board irrespective of the speed ofthe conveyor line or the length of the board. In addition, there is aneed for a controller able to control a gun for applying coatings ofvarying length and thickness to different zones of a single board.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system and methodfor applying a flux coating to a circuit board which obviates theproblems and limitations of the prior art systems.

It is a further object of the present invention to provide a system andmethod for applying an even solder flux oating to a circuit boardirrespective of the speed of the circuit board through the coatingchamber.

Yet another object is to provide a system and method for applying aneven coating of solder flux coating to a plurality of different lengthcircuit boards moving down a conveyor line.

Still another object is to provide a system and method for applying aflux coating to a circuit board with a pulsing pattern that pulses aspray gun on and off to coat a plurality of zones of a circuit boardwith different spray patterns.

In accordance with a first embodiment of the invention, a control systemfor controlling the application of a coating material to a substrate,such as a circuit board, and the method of operating the control systemincludes a conveyor for transporting the substrate in a direction oftravel. A spray gun emits a spray pattern of coating material. Anencoder disposed adjacent the conveyor outputs encoder pulse signalscorresponding to the distance of travel of the conveyor. A sensordisposed adjacent the conveyor and upstream of the spray gun a firstdistance senses the length of the substrate on the conveyor and outputsa length signal corresponding to a second distance between the leadingedge and trailing edge of the substrate. A controller which isoperatively connected to the spray gun, the encoder and the sensor, isactivated in response to the length signal from the sensor to output arecurring sequence of trigger signals for pulsing the spray gun on for aperiod of time and for a distance of travel to apply a relatively evencoating of material to the substrate irrespective of the speed of theconveyor or the length of the board.

In a second embodiment of the invention, the control system and methodof operating the control system includes a controller that outputs arecurring sequence of trigger signals for pulsing the spray gun on for aperiod of time and for a distance of travel to apply coating of materialat different rates to different regions of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the presently preferredembodiment of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a side, elevational schematic illustration of a system forapplying a flux coating with an airless spray gun operated by animproved controller onto a circuit board being transported by a conveyorthrough a coating chamber, in accordance with the invention;

FIGS. 2A, 2B, and 2C show the position of the spray gun with respect tothe underside of a circuit board shown in cross section at threeconsecutive on pulses of the gun; and

FIG. 3 shows a plan view of a plurality of circuit boards carried on apallet;

FIG. 4 shows a plan view of a circuit board sprayed by a pulse patternhaving three zones of different length receiving different amounts ofspray;

FIG. 5 shows a plan view of a plurality of circuit boards carried on apallet with each board being sprayed by a pulse pattern having threezones of different length receiving different amounts of spray;

FIG. 6 shows a plan view of a plurality of circuit boards carried on apallet a vacant space between two of the boards;

FIGS. 7A and 7B show encoder correlated plots of spray gun actuation forthe present invention; and

FIGS. 7C and 7D show encoder correlated plots of spray gun actuation forthe prior art controller.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a schematic illustration of system10 for applying a flux coating to a substrate such as a circuit board12. System 10 includes a conveyor 14 for transporting circuit board 12through a flux coating chamber 16 in a direction of travel indicated byarrow 18. A spray gun 20 having a nozzle 22 is disposed within coatingchamber 16 and emits a liquid spray pattern 24 from nozzle 22. A controlsystem 26 is operatively connected to spray gun 20 for intermittentlypulsing gun 20 on and off to coat overlapping sections of theundersurface of circuit board 12 in response to circuit board 12 movinga predetermined distance through coating chamber 16. An overspraycollection system 28, located within coating chamber 16, collectsoversprayed flux and exhausts it through a vent 30 from coating chamber16.

As in the prior art, flux coating chamber 16 is located between anassembly station (not shown), where circuit components 54 are mounted oncircuit board 12 with their leads 39 inserted into through-holes 41extending through board 12, as shown in FIGS. 2A, 2B, and 2C, and apreheat station (not shown) directly downstream from coating chamber 16where the board, subsequent to being coated with a solder flux, isheated to a predetermined temperature which prevents damage from a highthermal gradient created when the board is passed through a conventionalwave solder machine (not shown).

Flux coating chamber 16 is formed with a front wall 32, and a rear wall34 provided with an inlet opening 36 and an outlet opening 38,respectively, through which conveyor 14 passes. Conveyor 14 includesspaced coplanar conveyor chains 40 (only one being illustrated) whichmove in the direction of travel 18. Chains 40 each carry a plurality ofequally spaced tabs 42, each with a finger 44 at its bottom whichprojects inwardly from the chain to which it is secured so as to pointtowards the fingers associated with the tabs (not shown) carried by theother chain (not shown). Fingers 44 serve to engage opposite edges ofcircuit board 12. The chains 40 are driven jointly, by a variable-speedelectric motor (not shown). When chains 40 are driven in the directionindicated by the arrow 18, each circuit board 12 is carried through theflux coating chamber 16 and sprayed with a liquid spray pattern 24dispensed by nozzle 22 of spray gun 20. After leaving flux chamber 16through outlet opening 38, each circuit board 12 is transported througha preheat chamber (not shown) and then through the solder wave (notshown) for soldering.

Referring to FIGS. 1, 2A, 2B, and 2C, a conventional circuit board 12 iscomprised of a sheet of insulative material, i.e. epoxy resin, havingleading and trailing ends 46 and 48, respectively. A plurality ofthrough-holes 41 extend between the coplanar upper surface 50 andundersurface 52 of board 12. Each through-hole 41 is plated with a layer43 of metal that is joined to a pair of spaced metallized areas (notshown), located on the opposite surfaces 50 and 52. Metal paths (notshown) are provided on one or both of the surfaces 50 and 52 forselectively connecting the metallized areas.

Circuit components 54 are mounted on the upper side 50 of circuit board12 by means of leads 39 passing through the through-holes 41 andsoldered in place to assure a solid mechanical and electrical bondbetween each lead and the metal layer in each correspondingthrough-hole. The soldering is preferably done in an automatic fashion,such as by passing the circuit board 12 over a wave of molten solder ina wave soldering machine (not shown) so that the wave of solder contactsthe undersurface 52 of the board and is drawn up into each through-hole41 by wetting forces to bond the leads 39 to the board.

Prior to soldering, solder flux is applied to the undersurface 52 of thecircuit board 12 so that during the subsequent soldering operation, thesolder will wet the leads 39, the metal layer within the through-holes41 and the metallized areas. To assure a solid mechanical and electricalbond between the leads 39 and the board 12, flux is preferably sprayedinto the through-holes 41 after the leads are already in place.

A principle feature of this invention is the use of a spray gun 20disposed within coating chamber 16 to apply a coating material such assolder flux, preferably a low-solid, or no-clean flux. While theinvention is directed to the application of a flux material, it iswithin the scope of the invention to apply other types of liquid coatingmaterial. The spray gun 20 is an on/off device and sprays the samevolume of solder flux for the same time period in which it is open. Thespray gun 20 can be a Nordson Model A7A gun equipped with a NordsonCross-Cut® nozzle, manufactured by Nordson Corporation of Westlake,Ohio, the assignee of the present invention. Spray gun 20 uses a spraycoating technique called airless atomization whereby the liquid upstreamof the nozzle orifice is hydrostatically pressurized to expand after itexits the nozzle orifice into the air at ambient pressure so that theliquid stream expands and is atomized by the hydraulic force alone.Spray gun 20 is mounted on a gun mounting fixture (not shown) to varythe distance and to center the gun nozzle 22 with respect toundersurface 52 of circuit board 12.

Another aspect of the invention is that the solder flux is pulsed ontothe undersurface 52 of board 12 to ensure that the coating is uniformlyapplied across the undersurface 52 of the board without generating undueoverspray. Pulsing in combination with a high pressure spray alsoensures effective penetration of through-holes 41 in circuit board 12because the flux impacts each through-hole in board 12 from more thanone angle, since the board is moving and the gun 20 is fixed.

As illustrated in FIG. 1, board 12 is transferred in the direction ofarrow 18. Board 12 is sprayed with the solder flux by spray gun 20 whichin turn is controlled by control system 26. The control system 26includes a sensor 56 which is connected to a controller 60 by a line 58.A first distance between the sensor 56 and the nozzle 22 of spray gun 20is programmed into controller 60. More typically, the first distance isset between sensor 56 and the front edge 24A of spray pattern 24. Whenthe leading end 46 of circuit board 12 passes under sensor 56, thesensor is activated until the trailing edge 48 of the board passes thesensor as the board moves in the direction of travel 18. Sensor 56outputs a length signal through line 58 to controller 60 correspondingto a second distance equal to the length of board 12. Sensor 56 ispositioned upstream from the location of spray gun 20 to preventoverspray from spray gun 20 coating the sensor. Control system 26includes an encoder 62 disposed adjacent and preferably connecteddirectly to conveyor 14. Encoder 62 outputs a number of encoder pulse(ep) signals to controller 60 through line 64 corresponding to thedistance of travel of conveyor 14.

In a first embodiment of the invention, controller 60 accesses a pulsingprogram from a conventional means, such as a computer 66, through line68. The pulsing program enables an operator to input, the distancebetween the sensor 56 and the nozzle 22. If desired, the distance can bemeasured between the sensor 56 and the position where the forward edge24A of the spray pattern 24 contacts the underside of board 12. Also,the operator inputs a selected period of time for pulsing spray gun 20on, and a predetermined number of encoder pulse signals corresponding tothe distance between the locations on the circuit board 12 from when thespray gun is pulsed on and then pulsed on again. The predeterminednumber of encoder pulse signals are counted from whenever the spray gunis pulsed on.

During the operation of the first embodiment, the control system 26,after having the first distance between the sensor 56 and the forwardedge 24A of the spray pattern 24, the selected period of time and thepredetermined number of encoder pulse signals already inputted into thepulsing program as mentioned above, is activated when a circuit board 12is moved by conveyor 14 under sensor 56. Sensor 56 is then activated tosense the length of circuit board 12 carried on conveyor 14 and tooutput a length signal corresponding to a second distance between theleading edge 46 and the trailing edge 48 of the circuit board into line58. The length signal is inputted into controller 60 which in responsethereto outputs a trigger signal for pulsing the spray gun 20 on for theselected period of time after the leading edge 46 of circuit board 12moves the first distance. Then, after circuit board 12 travels adistance corresponding to the predetermined number of encoder pulsesignals counted from when spray gun 20 is pulsed on, controller 60outputs another trigger signal for the selected period of time and thecircuit board 12 moves a distance corresponding to the predeterminednumber of encoder pulse signals counted from when the latter triggersignal. This sequence of trigger signals is repeated until the circuitboard 12 moves a distance corresponding to the length signal beingoutput by the sensor 56 when the controller 60 stops outputting triggersignals until the next circuit board passes under sensor 56 to repeatthe sequence again.

A unique aspect of the present invention relates to the benefits ofcontrol system 26, i.e., that the circuit board 12 will be coated withthe same volume of solder flux per unit length irrespective of the speedof the conveyor 14 or the length of the circuit board. This can beunderstood by the comparison of the following two examples usingarbitrary distances and times.

In example 1 relating to the first embodiment of the present invention,conveyor 14 is set to move at 100 encoder pulses per second (ep/sec).Encoder 62 can be setup so that conveyor 14 moves 1 inch for every 100encoder pulses (ep). As shown in FIG. 2A and FIG. 7A, spray gun 20 ispulsed on for selected period of time of 50 milliseconds (0.050 sec)after the leading edge 46 of circuit board 12 moves the first distance,for example 600 ep (6 inches), from sensor 56 to the location where thefront edge 24A of spray pattern 24 contacts the underside 52 of theboard. The spray is thus on 0.050 sec and is repeated every 50 ep whilethe board moves 0.05 inches. The width of the spray pattern 24, i.e.,between side edge 24A and 24B is typically about 1.5 inches in width.Spray pattern 24 is diagrammatically shown to be wider for purposes ofillustration. The width of the coating applied to the undersurface ofthe circuit board 12 in this example is thus 1.55 inches. After thecircuit board 12 has moved a distance equal to 50 ep (0.5 inches) fromthe position of when the gun is initially pulsed on, spray gun 20 ispulsed on again for 0.05 sec and the cycle or sequence repeats every 50ep. The next spray pulse is pulsed on again and coats a second sectionfor 1.5 inches plus 0.05 inches of board travel which overlaps the firstsection. Since the side edge of the spray pulse is approximately 1.5inches in width and since the second spray pulse will be actuated for0.050 sec at 0.5 inches from the start of the first spray pulse, thefirst and second sections and will overlap by approximately 1.0 inches.

In example 2 relating to the first embodiment of the present invention,as shown in FIG. 7B, the conveyor is set to move at 200 ep/sec. Encoder62 remains setup so that conveyor 14 moves 1 inch for every 100 encoderpulses (ep). The spray gun 20 is pulsed on for selected period of timeof 50 milliseconds (0.050 sec) after the leading edge 46 of the circuitboard 12 moves the first distance, for example 600 ep (6 inches) fromsensor 56 to the front edge 24A of spray pattern 24. The spray is thuson while the board moves 0.10 inches. The width of the spray pattern 24,i.e., between side edge 24A and 24B is typically about 1.5 inches inwidth. The width of the coating applied to the undersurface of thecircuit board 12 in this example is thus 1.60 inches. After the circuitboard 12 has moved a distance equal to 50 ep (0.50 inches) from theposition of when the gun is initially pulsed on, spray gun 20 is pulsedon again for 0.050 sec and the cycle or sequence repeats every 50 ep.The next spray pulse is pulsed on again and coats a second section for1.5 inches plus 0.10 inches of board travel which overlaps the firstsection. Since the side edge of the spray pulse is approximately 1.5inches in width and since the second spray pulse will be actuated for0.050 sec at 0.5 inches from the start of the first spray pulse, thefirst and second sections will still overlap by approximately 1.0inches.

Using the prior art controller as described in U.S. Pat. No. 5,415,337,the controller outputs a trigger signal for pulsing spray gun 20 on fora selected number of encoder pulses and then triggers the spray gun offfor a certain number of counts from encoder 128. This continues for apreprogramed distance. The prior art control system will coat circuitboard 12 with a different volume of solder flux per unit lengthdepending on the speed of the conveyor. This can be understood by thecomparison of the following two examples using arbitrary distances andtimes similar to those used in describing the first embodiment above.

In a third example relating to the prior art controller, as shown inFIG. 7C, the encoder remains setup so that conveyor moves 1 inch forevery 100 encoder pulses (ep) and the conveyor is set to move at adesired speed, such as for example, 100 ep/sec or one inch per second.At this speed, the encoder generates an encoder pulse for every 0.01inches of conveyor movement. The gun is pulsed open for 5 ep or 0.05inches of conveyor movement. The period of time that the gun is open is0.050 seconds. Then the controller closes the gun for 50 ep or 0.5inches of board movement. The width of the spray pattern between sideedge 24A and 24B is 1.5 inches in width. The width of the coatingapplied to a first section of the undersurface of the circuit board 12in this example is thus 1.55 inches. After the circuit board 12 hasmoved a distance equal to 50 ep (0.5 inches) from the position of whenthe gun is pulsed off, spray gun 20 is pulsed on again for 5 ep or 0.05inches of conveyor movement coats a second section for 1.5 inches plus0.05 inches of board travel which overlaps the first section. Then, thegun is pulsed off for 50 ep and the cycle or sequence repeats itselfuntil the end of the board is sensed by the sensor. Since the side edgeof the spray pulse is approximately 1.5 inches in width and since thesecond spray pulse will be actuated for 0.05 inches, after the start ofthe first spray pulse, the first and second sections will still overlapby approximately 1.0 inches.

In a fourth example relating to the prior art controller, as shown inFIG. 7D, the encoder remains setup so that conveyor moves 1 inch forevery 100 encoder pulses (ep) and the conveyor is set to move at adesired speed, such as for example, 200 ep/sec or two inches per second.The encoder generates an encoder pulse for every 0.01 inches of conveyormovement. The gun is pulsed open for 5 ep or 0.05 inches of conveyormovement. Note, however that the gun is only on for 0.025 sec ascompared with 0.050 sec in Example 3. Since the volume of solder fluxdispensed by the gun in the same period of time does not change, inExample 4 about one half of the volume of solder flux is coated onto thesame length of board as coated onto the board of Example 3 where theboard is moving at half the speed of the board in Example 4. The priorart controller continues the sequence by closing the gun for 50 ep or0.5 inches of board movement. The width of the spray pattern betweenside edge 24A and 24B is 1.5 inches in width. The width of the coatingapplied to a first section of the undersurface of the circuit board 12in this example is thus 1.55 inches. After the circuit board 12 hasmoved a distance equal to 50 ep (0.5 inches) from the position of whenthe gun is pulsed off, spray gun 20 is pulsed on again for 5 ep or 0.05inches of conveyor movement coats a second section for 1.5 inches plus0.05 inches of board travel which overlaps the first section. Then, thegun is pulsed off for 50 ep and the cycle or sequence repeats itselfuntil the end of the preprogramed distance. Since the side edge of thespray pulse is approximately 1.5 inches in width and since the secondspray pulse will be actuated for 0.05 inches, after the start of thefirst spray pulse, the first and second sections will still overlap byapproximately 1.0 inches. However because of the increased speed of theconveyor, only one half of the amount of solder flux will be coated tothe underside of the board.

Another aspect of the first embodiment is the ability to program in thepulsing program a lead distance of a desired number of encoder pulseswhich represent an additional distance that the circuit board travelsbefore the spray gun is pulsed on. This feature is useful to account forthe width of the spray pattern so that the spray does not extend pastthe leading edge 46 of board 12 as overspray. Also, this feature isuseful when several circuit boards 12 are carried on a pallet 70, asillustrated in FIG. 3. Pallet 70 has a front rail 72 and a rear rail 74and the circuit boards 12 are abutted against each other therebetween.In order that the front rail 72 is not sprayed, the lead distance isprogrammed into the pulse program so that the gun is not pulsed on untilthe front rail of pallet 70 moves a number of encoder pulses equal toboth the predetermined distance between sensor 56 and the nozzle 22 plusthe width of front rail 72. In addition, the pulsing program can includea lag distance of a desired number of encoder pulses which represent thewidth of rear rail 74. In order that the rear rail 74 is not sprayed,the lag distance is programmed into the pulse program so that gun 20 isnot pulsed on when pallet 70 moves a second distance between leadingedge 46 and trailing edge 48 of circuit board 12 less the lag distance.

Another feature of the first embodiment is the provision of a minimumtrigger distance in encoder pulses. If the sensor 56 turns on or off forthe minimum trigger distance, the signal is ignored and does not effectthe control function of controller 60. For example, if a circuit board12 has an open area whose length is less than the minimum triggerdistance, the signal from sensor 56 that would indicate the presence ofthe trailing edge is less than the minimum trigger distance and does notactivate the controller 60.

In accordance with a second embodiment of the invention, controller 60accesses a pulsing program with a pattern control mode from aconventional means, such as a computer 66, through line 68. The patterncontrol mode pulsing program enables an operator to input a pulsingpattern of a plurality of zones each having a different pulse pattern(period of time that the gun is pulsed on and predetermined number ofencoder pulse signals for the length of each zone) to be sprayed on thecircuit board. After the pulsing pattern is sprayed, the pulsing patternrepeats itself until the end of the board. While in a typicalapplication the pulsing pattern includes two or three different zonesprogrammed into the pattern control mode pulsing program, it is withinthe terms of the present invention to program in addition zones thepulsing pattern as required. The pattern control mode pulsing programalso enables an operator to input a predetermined number of encoderpulse signals corresponding to a second distance between leading edge46' and trailing edge 48' of circuit board 12', as shown in FIG. 4.Throughout the specification, primed numbers represent structuralelements which are substantially identical to structural elementsrepresented by the same unprimed number.

During the operation of the second embodiment, the control system 26,after having the first distance between the sensor 56 and the nozzle 22or the forward edge 24A of the spray pattern 24, the selected period ofpulse on time and the predetermined number of encoder pulse signals foreach plurality of zones A, B, C, as shown in FIG. 4, of the pulsingpattern already inputted into the pulsing program as mentioned above, isactivated when conveyor 14 moves a circuit board 12' under sensor 56.Sensor 56 is then activated to sense the leading edge 46' of the circuitboard 12' carried on conveyor 14 and to output a sensor signal into line58. The sensor signal is inputted into controller 60 which in responsethereto outputs a first trigger signal for pulsing the spray gun 20 onfor a first period of time during the movement of circuit board 12' fora first predetermined number of encoder pulses corresponding to thelength of the first zone A, then a second trigger signal for pulsing thespray gun 20 on for a second period of time during the movement of thecircuit board for a second predetermined number of encoder pulsescorresponding to the length of the second zone B, then a third triggersignal for pulsing the spray gun 20 on for a third period of time duringthe movement of the circuit board for a third predetermined number ofencoder pulses corresponding to the length of the third zone C. Then theprogram repeats the sequence and applies the pulsing pattern for thenext circuit board passing under sensor 56.

Another aspect of the second embodiment is the ability to program in thepulsing program a lead distance of a desired number of encoder pulseswhich represent an additional distance that circuit board 12' travelsbefore spray gun 20 is pulsed on. This feature is useful to account forthe width of the spray pattern so that the spray does not cross theleading edge 46' of the board 12' as overspray. Also, this feature isuseful when several circuit boards 12' are carried on a pallet 70', asillustrated in FIG. 5. Pallet 70' has a front rail 72' and a rear rail74' and the circuit boards 12' are abutted against each othertherebetween. In order that front rail 72' is not sprayed, the leaddistance is programmed into the pattern control mode pulsing program sothat the spray gun is not pulsed on until the front rail 72' of pallet70' moves a number of encoder pulses equal to both the predetermineddistance from the sensor 56 to nozzle 22 plus the width of the frontrail. In addition, the pattern control mode pulsing program can includea lag distance of a desired number of encoder pulses which represent thewidth of rear rail 74'. In order that the rear rail 74' is not sprayed,the lag distance is programmed into the pulse program so that the gun isnot pulsed on when pallet 70' moves a second distance between theleading edge 46' and trailing edge 48' of circuit board 12' less the lagdistance.

Another feature of the pattern control mode pulsing program is theprovision of a minimum trigger distance in encoder pulses. If sensor 56turns on or off for the minimum trigger distance, the signal is ignoredand does not effect the control function of controller 60. For example,if the circuit boards 12' are loaded on pallet 70' so that a vacantspace 76 exists between the trailing edge 48' of circuit board 12' andthe leading edge 46' of the adjacent circuit board, as shown in FIG. 6,the controller does not stop pulsing the gun on because the length ofthe vacant space is less than the minimum trigger distance. In anotherexample, when the width of the rear rail 74' is less than the minimumtrigger distance, the signal from sensor 56 is less than the minimumtrigger distance and does not activate the controller 60.

It is apparent that there has been provided in accordance with thisinvention a system and method for applying a flux coating to a circuitboard that satisfy the objects, means and advantages set forthhereinbefore. According to one embodiment, a control system operates aspray gun to apply an even coating of flux to a circuit board or aplurality of different length circuit boards irrespective of the speedthat the circuit board(s) move through the coating chamber. In a secondembodiment, the control system pulses the spray gun on and off with apulsing pattern that coats a plurality of zones of a circuit board withspray patterns of flux of different length and thickness.

While the invention has been described in combination with embodimentsthereof, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art in light of theforegoing teachings. Accordingly, the invention is intended to embraceall such alternatives, modifications and variations as fall within thespirit and scope of the appended claims.

We claim:
 1. A system for controlling the application of a coatingmaterial onto a substrate, said system comprising:a conveyor fortransporting said substrate in a direction of travel; a spray gunemitting a spray pattern of coating material for application to saidsubstrate; an encoder disposed adjacent said conveyor to output encoderpulse signals corresponding to the distance of travel of said conveyor;a sensor disposed adjacent said conveyor and upstream of said spray guna first distance to sense said substrate on said conveyor measure theunknown length of said substrate and output a sensor signal indicativeof a length of said substrate; and a controller operatively connected tosaid spray gun, said encoder and said sensor, said controller beingactivated in response to said sensor signal to output trigger signalsfor pulsing said spray gun on to apply material at different rates todifferent regions of said substrate.
 2. A system for controlling theapplication of a coating material onto a substrate having a leadingedge, a trailing edge and being of an unknown length, said systemcomprising:a conveyor for transporting said substrate in a direction oftravel; a spray gun disposed adjacent said conveyor, said spray gunemitting a spray pattern of coating material for application to saidsubstrate; an encoder disposed adjacent said conveyor for outputting anumber of pulse signals correlated to the distance of travel of saidconveyor; a sensor disposed adjacent to said conveyor and upstream ofsaid spray gun a first distance to sense said substrate on said conveyorand measure the unknown length of said substrate and output a sensorsignal corresponding to said unknown length of said substrate; and acontroller operatively connected to said spray gun, said encoder andsaid sensor, said controller being activated in response to said sensorsignal to output a recurring sequence of at least two patterns oftrigger signals after the leading edge of said substrate moves saidfirst distance, the first of said patterns of trigger signals pulsingsaid spray gun on for a first period of time beginning when saidsubstrate travels a second distance equal to a first number of encoderpulse signals and the second of said at least two patterns of triggersignals pulsing said spray gun on for a second period of time when saidsubstrate travels a second distance equal to a second number of encoderpulse signals counted from the last of said first number of encoderpulses.
 3. A system for controlling the application of a coatingmaterial to a substrate having a leading edge, a trailing edge and beingof an unknown length, said system comprising:a conveyor for transportingsaid substrate in a direction of travel; a spray gun disposed adjacentto said conveyor, said spray gun emitting a spray pattern of coatingmaterial for application to said substrate; an encoder disposed adjacentsaid conveyor for outputting a number of pulse signals correlated to thedistance of travel of said conveyor; a sensor disposed adjacent to saidconveyor and upstream of said spray gun a first distance for measuringthe unknown length of said substrate on said conveyor and for outputtinga length signal corresponding to said unknown length of said substrate;and a controller operatively connected to said spray gun, said encoderand said sensor, said controller receiving said length signal from saidsensor and said pulse signals from said encoder and outputting to saidspray gun a first and second plurality of trigger signals, said firstplurality of trigger signals being outputted during a time periodcorrelated to the length of a first portion of said substrate andstarting after said leading edge of said substrate moves said firstdistance, each of said first plurality of trigger signals starting aftersaid substrate moves a first predetermined distance and lasting a firstpredetermined length of time, and said second plurality of triggersignals being outputted during a time period correlated to the length ofa second portion of said substrate and starting after said leading edgeof said substrate moves a distance equal to said first portion of saidsubstrate, each of said second plurality of trigger signals startingafter said substrate moves a second predetermined distance and lasting asecond predetermined length of time.
 4. The system as set forth in claim3 wherein said first predetermined length of time is equal to saidsecond predetermined length of time and said first predetermineddistance is not equal to said second predetermined distance.
 5. Thesystem as set forth in claim 3 wherein said first predetermined lengthof time is not equal to said second predetermined length of time andsaid first predetermined distance is equal to said second predetermineddistance.
 6. The system as set forth in claim 3 wherein said firstpredetermined length of time is not equal to said second predeterminedlength of time and said first predetermined distance is not equal tosaid second predetermined distance.
 7. The system of claim 3 whereinsaid spray gun is on only during the receipt of each of said first andsecond plurality of trigger signals.
 8. The system of claim 3 whereinsaid first plurality of trigger signals start after said leading edge ofsaid substrate moves said first distance plus an additional distance. 9.The system of claim 8 wherein said second plurality of trigger signalsend after said leading edge of said substrate moves a distance equal tosaid length of said substrate minus an additional distance.
 10. Thesystem of claim 3 wherein said first plurality of trigger signals endafter said leading edge of said substrate moves a distance equal to saidlength of said first portion plus an additional distance.
 11. The systemof claim 3 wherein said second plurality of trigger signals end aftersaid leading edge of said substrate moves a distance equal to saidlength of said substrate minus an additional distance.